The present invention relates to a method of detecting a focus condition of an imaging optical system such as a camera, a microscope, a high density optical recording and reproducing apparatus or the like.
Two methods have been developed for detecting an in-focused condition of an object image formed by an imaging optical system, one being an image sharpness detection method and the other being an image lateral shift detection method. Each of these methods has not only superior merit, but also a defect so that the use of only one method cannot attain a detection of focus condition with high precision. That is, the image sharpness detection method has a poor detection sensitivity to an image including a low space frequency component and the image lateral shift detection method has a poor detection sensitivity to a low contrast image (the image having small difference between the maximum value and the minimum value of contrast intensity). The image sharpness detection method also has a defect that the direction of front focus and rear focus cannot be detected in the position sufficiently apart from the in-focused position.
In order to overcome the above defects, the present applicant provides an apparatus for detecting focus condition by combining the above two detection methods in Japanese Patent Laid-open No. 169,617/82 and Japanese Patent Application No. 95,869/82. In these apparatuses both detection methods are combined. That is, the image lateral shift detection method is utilized in the large defocused region and in the small defocused region the focus condition is detected by only the image sharpness detection method or a combination of the image sharpness detection method and the image lateral shift detecting method. For example, consider FIGS. 1a and 1b, wherein the inclination of the photoelectric converted outputs of light incident upon a light receiving element array used for image sharpness detection is large as in FIG. 1a and the inclination of the photoelectric converted outputs is small as in FIG. 1b. In the large inclination as shown in FIG. 1a, the conventional method can be utilized because of the large difference between the in-focused condition and the defocused condition. In the small inclination as shown in FIG. 1b, the focus condition cannot be detected by the image sharpness detection method because of the very small difference between the in-focused condition and the defocused condition. FIGS. 2a and 2b are graphs showing the evaluation function values of the image, providing that F.sub.1 or ##EQU1## and ##EQU2## wherein F.sub.1 and F.sub.2 are evaluation function values obtained by the image sharpness method, F.sub.3 is an evaluation function value obtained by the lateral shift method, Mj is a maximum absolute value of the difference between outputs of adjacent light receiving elements j, and A.sub.i and B.sub.i are outputs of respective light receiving elements in the light receiving element array which receives the lights passing through one and the other imaging lenses. In FIGS. 2a, and 2b, when the inclination of evaluation value is small as shown in FIG. 1b, evaluation values F.sub.1 and F.sub.2 of the defocused image are larger than the threshold value B as shown in FIG. 2a while the difference of these evaluation values is small, so that the evaluation function F.sub.1 -F.sub.2 for focusing detection is placed between the threshold value .+-..DELTA..sub.1 for focus detection and thus the display width of focusing condition becomes larger than the focus depth resulting in the incapability of deciding precise focusing. However, in the image lateral shift detection method, as shown in FIGS. 3a and 3b by the output of elements in light receiving element arrays A and B, laterally shifted amounts of image between arrays A and B are substantially not changed for the inclination of photoelectric converted outputs so that the focusing decision by the image lateral shift detection method can be effected with sufficient precision in the same manner as the conventional means as shown in FIG. 2b.